1. Field of the Invention
The present invention relates to a method of molten metal plating and an apparatus therefor.
Steel strips plated with Zn, Al, Sn, or Pb or alloys thereof are widely applied for automobiles, architecture, electric equipment, and cans and improved quality and production efficiency is desired.
2. Description of the Related Art
A conventional method of molten metal plating comprises: heating a steel strip in a reducing atmosphere to clean the surface thereof; directing the strip into a bath of a molten metal to be deposited; lifting the strip deposited with the metal out of the bath; and then immediately subjecting the strip to a gas sprayed from a slit-shaped nozzle to remove an excess deposit metal and thereby control the deposited metal amount. Another conventional method brings a steel strip into contact with a molten metal only on one side thereof and the deposited metal amount is controlled in the same way.
This hot-dip plating is applied in the production of blank materials currently widely used, typically in Zn-plating, Al-plating, and turn plating.
The hot-dip plating has a disadvantage in that a strip is partially dissolved in a plating bath when the strip passes through the bath and most of the dissolved iron from the strip forms an intermetallic compound with the bath components and floats in the bath as a floating dross. The dross is entrained in the plated layer during plating process and degrades the appearance, corrosion resistance and formability of a plated product.
Another disadvantage is the plating bath which must have a large volume sufficient to introduce and dip a steel strip therein by using a pot roll. To change the composition of such a large volume of plating bath, particularly when the kind of product is to be changed, it is necessary to bail out part of the bath and replenish or add a plating metal or an additive metal. This requires a lot of cost, time and labor and only a limited kind of product can be processed in the same plating line.
Another disadvantage is that dipping requires a long time causing a reaction between a steel strip and a plating metal to form a thick brittle alloy layer which impairs the formability of the plated product. Additives are fed to the plating bath to reduce the thickness of the alloy layer, but this measure becomes insufficient when the plated products are subjected to heavier forming.
Moreover, the ambient atmospheric oxygen reacts with the molten metal to generate an oxidized dross causing an undesirable consumption of the metal bath, depositing on the strip surface and thereby imparing the product appearance.
The most general method of controlling the deposited metal amount is the above-mentioned gas spray. When a line speed is 160 m/min or higher, the excess metal removed from a steel strip violently splashes and adheres again to the strip, and the amount of metal lifted by the strip and the amount of generated dross are also increased. The line speed is therefore limited.
To solve the above-mentioned problems, U.S. Pat. No. 3,201,275 proposed a method in which a resin solution is sucked up by capillarity from a level lower than a coating nozzle to form a meniscus of the solution on the coating nozzle and the meniscus is brought into contact with a tape to apply the solution on the tape. When this method is used in molten metal plating, the following problems arise. To ensure a satisfactory suction of a molten metal by capillarity, a suction pipe must be made of a material having a good wettability with the molten metal. Such a material, however, also easily reacts with the molten metal and thereby causes contamination of the molten metal during the suction and blockage of the pipe. Moreover, a molten metal has a specific gravity greater than that of a resin solution and is difficult to suck stably, with the result that when the travelling speed of a metal strip is high the molten metal supply is insufficient to ensure a good coating. The high speed travelling of a metal strip also has a problem in that the ambient gas, dragged by the travelling strip, collides with the meniscus at a high speed and is engulfed in the meniscus, to cause the formation of a discrete coating which is not practically applicable.
Japanese Unexamined Patent Publication (Kokai) No. 61-207555 proposed a method capable of solving the above-mentioned problem of an insufficient supply of a molten metal, in which method a meniscus of a molten metal is formed on the outlet opening of a nozzle and a metal strip is brought into contact with the meniscus while travelling. This increases the outflow of the molten from the outlet opening in comparison with an outflow of a metal freely flowing out of the opening and the deposited metal amount can be easily controlled. This increase in outflow is caused by the wetting adhesion of the molten metal to the strip and the deposited metal amount is controlled to a constant value in accordance with the travelling speed of the metal strip. When the control of the deposited metal amount is effected by adjusting the distance between the nozzle outlet opening and the metal strip, there is a tendency for the deposited metal amount to abruptly change at a certain value of the distance and does not significantly vary at distances greater or smaller than this value. To ensure stable control, the distance need be set at a value not causing a significant change in the deposited metal amount, and therefore, a desired deposited metal amount is not always obtained.
To solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 61-235550 proposed a method in which a dam is provided within the opening of a plating nozzle to provide a constant gap at the dam and partially close the opening or decrease the sectional area for the passage of a molten metal, whereby the amount of sucked molten metal is controlled. Specifically, the dam is composed of a plurality of members respectively slidable in the gap-ward direction and part of the members are moved down towards the gap at a constant interval.
This method, however, has a disadvantage in that the flowout speed is difficult to control precisely and uniformly over the width of a metal strip to be plated and that the nozzle gap of 0.6 mm varies because of thermal distortion, etc., to cause a non-uniform deposited metal amount over the strip width, which cannot be restored by any means. Therefore, this method cannot be applied to practical use. Likewise in the previously recited U.S. Pat. No. 3,201,275, the high speed travelling of a metal strip also has a problem in that the ambient gas, dragged by the travelling strip, collides with the meniscus at a high speed and is engulfed in the meniscus, to cause the formation of a discrete coating which is not practically applicable.
Japanese Unexamined Patent Publication (Kokai) No. 59-67357 disclosed a method based on a production process of amorphous ribbons, in which a molten metal is sprayed on a travelling steel strip, instead of a rotating disc, either through a slit-shaped nozzle or a multiple opening nozzle and the sprayed molten metal is cooled by the steel strip to form a metal coating on the strip. Specifically, a vessel containing a molten metal and having a slit-shaped nozzle or a multiple opening nozzle, is disposed above a steel strip travelling on a drum, with the nozzle tip being close to the strip, usually at a distance of not more than 1 mm. The flowout speed of the molten metal is controlled either by the level of the nozzle head or by a pressure of an inert gas such as argon.
This method also has a problem in that a non-uniform flowout speed over the strip width directly causes a non-uniform deposited metal amount over the strip width and a widthwise control of the flowout speed is essentially important to ensure a uniform plating over the strip width, but such a control is not disclosed for practical application. Likewise in the previously recited methods, the high speed travelling of a metal strip also has a problem in that the ambient gas, dragged by the travelling strip, collides with the meniscus at high speed and is engulfed in the meniscus, to cause the formation of a discrete coating which is not practically applicable, and therefore the travelling speed is limited.
This problem due to the high travelling speed is also experienced in the extrusion of a molten resin by "T-die" process, and in the same manner as in this process, the ambient atmospheric gas may be evacuated to provide a vacuum. In a continuous production line, however, expensive equipment such as a differential evacuation system is required and the evacuation capacity must be increased when using a high travelling speed, which cannot practically be applied.
The above-recited conventional methods commonly have the following problems.
A problem arises when a metal strip is plated on both sides thereof. In a method in which a fluctuation of the strip passage line, including the vibration of a travelling strip, is suppressed by support rolls and the strip is plated first on one side and then on the other side by using a nozzle disposed near the strip, the side to be later plated is brought into contact with the support rolls upon plating. In molten metal plating processes, a steel strip is maintained at a temperature near the melting point of a plating metal, and therefore, the metal deposited on the first side of the strip is in a molten or semi-molten state and the contact with the support rolls causes a non-uniform appearance and quality.
Another problem resides in continuous productivity. Continuous production requires that strip coils be bonded with each other by welding, the welded joint has an uneven profile along the strip width due to thermal distortion and collides with the plating nozzle disposed near the strip. The collision may be avoided by retreating the nozzle, but it is not actually possible to move the nozzle at a precision of several to several tens of micrometers together with the associated heavy equipment including a molten metal pot, a runner, etc. Moreover, a steel strip to be plated may not have a flat shape but may have corrugations across the width or length of the strip. Such a strip shape also makes it difficult to stably maintain a constant distance between the strip and a plating nozzle.